This invention relates to measurement device for use on coordinate measuring apparatus, such as a coordinate measuring machines (CMMs) machine tools, manual coordinate measuring arms and inspection robots. More particularly, it relates to signal transmission systems for such measurement devices.
A trigger probe for determining the position of a workpiece is described in U.S. Pat. No. 4,153,998. In use, the probe is moved by the machine relative to a workpiece. The probe has a deflectable stylus and delivers a trigger signal when the stylus contacts the workpiece. The trigger signal is indicated by the probe switching from one state to another. The trigger signal is used by the machine controller to freeze the outputs of scales or other measuring means which indicate the position of the probe. The position of the point of contact on the workpiece surface can thus be determined.
Especially on machine tools, it can be difficult to wire the probe directly to the machine controller, and so various wireless signal transmission systems have been developed in the prior art. These include inductive systems (where the signal is transmitted by electromagnetic induction between two coils), optical systems (where an optical emitter such as an infra-red diode is provided on the probe and produces an optical signal which is received by an appropriate receiver) and radio systems (having a radio transmitter in the probe and a radio receiver fixed at a convenient location on the machine). An example of a radio system is shown in U.S. Pat. No. 4,119,871. An important requirement of such probes is repeatability, i.e. that the same result should be achieved every time a given measurement is repeated. The mechanical position of the stylus in the probes described in U.S. Pat. No. 4,153,998 is extremely repeatable in space, an instant of production of the trigger signal always has a definite, repeatable relationship with the instant of contact between the stylus and the workpiece. This means that accurate results can be obtained from the probe by a simple calibration procedure.
However, the accuracy would be destroyed if the signal transmission system were not repeatable, that is, if there were an unknown, variable delay in the signal transmission. If this occurs then the probe would travel an unknown variable distance after the instant of generation of the trigger signal before the machine control is able to freeze the outputs of the measuring means. There is then an error between the position of contact and the position indicated by the frozen outputs and this error is an unknown variable quantity which cannot be removed by calibration. Thus, in order to maintain overall accuracy of the probe system there is the problem of ensuring that any transmissions delays introduced by the signal transmission system are repeatable i.e. the same delay should be introduced every time the probe is triggered. The probe calibration procedure mentioned above will then also remove this repeatable delay caused by the transmission system.
U.S. Pat. No. 5,279,042 discloses an analogue radio signal transmission system for a probe in which the probe is provided with a transmitter for producing a carrier signal onto which a probe signal may be modulated. A receiver receives the probe data and produces a probe output signal derived from the transmitter data. A clock on the transmitter provides a time standard for the whole system, the receiver uses an oscillator with a phase comparator at its input to ensure that the oscillator is permanently synchronised with a clock in the transmitter. When a probe signal occurs, the time elapsed between the start of a counter cycle and the change of state of the probe is latched in a shift register and transmitted serially.